1. Field of the Invention
The present invention relates to a solenoid for fuel injection, which is used in an electronically controlled fuel injection device that supplies fuel to an engine or the like. More particularly, the present invention relates to a solenoid driving device using a system in which the electric power that is accumulated in the solenoid when the driving of the solenoid is stopped is temporarily stored in a capacitor, and the electric power that is stored in this capacitor is supplied to the solenoid when the solenoid is again driven.
2. Description of the Related Art
FIG. 8 is a block diagram which shows the construction of a common solenoid driving device. This solenoid driving device is constructed from a solenoid 11, a solenoid driving element 12 which is used to drive the solenoid 11, a solenoid driving element control circuit 13 which controls the on/off switching of the solenoid driving element 12 on the basis of control signals that are input from the outside (i.e., externally supplied), and a snubber circuit 14 that is used to consume the electric power that is accumulated in the solenoid 11 when the driving of the solenoid 11 is stopped. In FIG. 8, reference numeral 15 indicates a power supply terminal to which a power supply voltage (battery voltage) VB is applied, and reference numeral 16 indicates a control signal input terminal.
In the case of the solenoid driving device constructed as shown in FIG. 8, when the solenoid driving element 12 is in an “on” state, current flows through the solenoid 11, and fuel is injected after a fixed period of time. After a fixed period of time has elapsed in this state, the solenoid driving element 12 is switched to an “off” state in order to stop the injection of fuel. At this time, the current that had been flowing to the solenoid 11 flows to the snubber circuit 14, and the electric power is consumed by this snubber circuit 14. As a result, the electric current that flows to the solenoid 11 is gradually reduced, and eventually reaches zero so that the injection of fuel is stopped.
FIG. 9 is a circuit diagram which shows the concrete construction of the solenoid driving device shown in FIG. 8. The solenoid driving element 12 is constructed from an N-channel field effect transistor (hereafter referred to as an “FET”) 121. The solenoid driving element control circuit 13 is constructed from an npn transistor 131 and four resistors 132, 133, 134 and 135. The snubber circuit 14 is constructed from a Zener diode 141.
One end of the solenoid 11 is connected to the power supply terminal 15, and the other end of the solenoid 11 is connected to the drain terminal of the FET 121 and the cathode terminal of the Zener diode 141. The source terminal of the FET 121 and the anode terminal of the Zener diode 141 are grounded. The collector terminal of the npn transistor 131 is connected to the gate terminal of the FET 121. The first resistor 132 is connected between the collector terminal of the npn transistor 131 and the power supply terminal 15. The base terminal of the npn transistor 131 is connected to the control signal input terminal 16 via the second resistor 133. The control signal input terminal 16 is pulled up to the power supply voltage Vcc by the third resistor 134. The emitter terminal of the npn transistor 131 is connected to the base terminal via the fourth resistor 135, and is grounded.
FIG. 10 is a circuit diagram which shows another example of the concrete construction of the solenoid driving device shown in FIG. 8. In the solenoid driving device shown in FIG. 10, the anode terminal of the Zener diode 141 in the device shown in FIG. 9 is connected to the collector terminal of the npn transistor 131 via a fifth resistor 136 instead of being grounded, and a diode 142 is connected between the cathode terminal of the Zener diode 141 and the solenoid 11 so that the diode 142 is oriented in the direction in which current flows from the solenoid 11 to the Zener diode 141.
However, in the case of the solenoid driving devices constructed as shown in FIGS. 8 through 10, when the capacity of the solenoid 11 is increased, the electric power that is consumed by the snubber circuit 14 is considerably increased, and as a result, the generation of heat becomes a problem. Accordingly, for the purpose of reducing this generation of heat and achieving effective utilization of the electric power and an increased driving speed, a solenoid device is universally known with a construction in which the energy that accumulates when the coil current that flows to the solenoid is stopped is temporarily stored in a capacitor, and the coil current is abruptly increased by utilizing the energy stored in the capacitor when the coil current is again caused to flow to the solenoid.
FIG. 11 is a block diagram which shows the construction of a conventional solenoid driving device of the type in which energy stored in a capacitor is utilized in the re-driving of the solenoid. This solenoid driving device is constructed from a solenoid 11, a solenoid driving element 12, a solenoid driving element control circuit 13, a capacitor 21 that temporarily stores the energy that accumulates when the driving of the solenoid 11 is stopped, a discharge control element 22 that controls the discharge of the capacitor 21, a discharge control circuit 23 that controls the on/off switching of the discharge control element 22, a DC—DC converter circuit 24 that raises the power supply voltage VB and supplies a high voltage to the discharge control circuit 23, a current back-flow preventing circuit 25 that prevents the voltage from entering the power supply side when a high voltage stored in the capacitor 21 is applied to the solenoid 11, and a rectifying element 26 that prevents a direct current from flowing into the solenoid driving element 12 from the capacitor 21 as a result of the high voltage stored in the capacitor 21. Furthermore, constructions that are the same as in the apparatus shown in FIG. 8 are labeled with the same symbols, and a description thereof is omitted.
Next, the operation of the solenoid driving device constructed as shown in FIG. 11 will be described. First, when the solenoid driving element 12 is switched from an “off” state to an “on” state by the control of the solenoid driving element control circuit 13, a current begins to flow to the solenoid 11 from the power supply terminal 15 via the current back-flow preventing circuit 25. Then, after a fixed period of time has elapsed, fuel injection is initiated. After another fixed period of time has elapsed, the solenoid driving element 12 is switched to an “off” state in order to stop the injection of fuel. At this time, the current that had been flowing to the solenoid 11 flows to the capacitor 21 via the rectifying element 26. The voltage VC of the capacitor 21 rises at the same time that current flows in, so that the electric power that had accumulated in the solenoid 11 is absorbed by the capacitor 21. The rise of the voltage VC of the capacitor 21 stops at the same time that the current flowing into the capacitor 21 reaches zero.
When fuel injection is again performed following this state, the solenoid driving element 12 is switched to an “on” state, and at the same time, the discharge control element 22 is switched to an “on” state. As a result, the voltage VSH on the high-potential side of the solenoid 11 becomes the same as the voltage VC generated by the charging of the capacitor 21, and becomes higher than the power supply voltage VB. Accordingly, a current abruptly begins to flow to the solenoid 11. Since this current flows out from the capacitor 21, the voltage VC of the capacitor 21, i.e., the voltage VSH on the high-potential side of the solenoid 11, drops. Then, at the point in time at which the voltage VSH on the high-potential side of the solenoid 11 becomes lower than the power supply voltage VB, the current that flows out of the capacitor 21 becomes zero, and a current begins to flow to the solenoid 11 from the power supply voltage VB. In this case, the current that flows to the solenoid 11 continues to increase to the voltage that is limited by the winding resistance of the solenoid 11.
Thus, excluding the initial fuel injection, the current that flows to the solenoid 11 is abruptly increased by the voltage that is generated by the charging of the capacitor 21 during the second and subsequent fuel injections. During this abrupt increase, the current that flows from the power supply terminal 15 is zero. Accordingly, the amount of current that flows from the power supply terminal 15 is decreased overall, so that the power consumption is reduced. Furthermore, the current that flows through the solenoid 11 abruptly rises to a value that is close to the required current, so that the response is improved.
However, in the abovementioned conventional solenoid driving device of the type in which energy stored in capacitor is utilized for re-driving of the solenoid, as is shown in FIG. 11, a DC—DC converter circuit 24, which is used to supply a high voltage to the discharge control circuit 23, is necessary, resulting in the problem of an increased complexity of the circuit and an increased size of the circuit. Furthermore, since the current back-flow preventing circuit 25 is ordinarily constructed from a diode, the following problem also arises: namely, when a large power supply current flows through this circuit, the amount of heat generated is increased as a result of the voltage drop of approximately 0.7 V of the diode. Furthermore, since the rectifying element 26 between the solenoid driving element 12 and the capacitor 21 is also constructed from a diode, the generation of heat caused by the current that flows through this diode is also a problem.